This invention relates to metal articles which have decorative chromium plating in layers of the order of 0.002 to 0.10 mil over a metal substrate such as nickel, nickel-iron or cobalt plate with or without other plate layers between the substrate and a metal base which is usually a ferrous metal. The thinness of the chromium layer allows the quality of the substrate surface to affect the total surface appearance. Thus, if the substrate has a bright surface, the chromium-plated substrate will also have a bright surface. If the substrate has a semi-bright or lustrous surface, the chromium-plated substrate will also have a semi-bright or lustrous surface. However, such plating fails to protect the metal base from corrosion which develops at small cracks and/or single pores in the chromium layer and under corrosive conditions reaches the metal base. While thicker layers of chromium plate and/or substrate give better protection to the base, the additional cost is unattractive, the resulting plate is not decorative, and such thicker layers do not guarantee against such cracks because the presence of few corrosion sites in any thickness plate leads to severe attack of the substrate down to the metal base.
The presence of many microcracks and/or micropores in the exposed chromium plate layer apparently creates a stable condition wherein no major deep corrosion of the substrate takes place, thereby protecting the metal base under the substrate from any corrosion whatever. Microcracks have been produced by inducing high stress in the chromium and/or substrate layers whereby the microcrack pattern develops to relieve the resulting strain. Such stress is most commonly produced by including ions such as the selenate or fluosilicate ions in chromium plating baths and ammonium and chloride ions with embrittling agents in nickel plating baths (U.S. Pat. No. 3,563,864). Micropores have been produced mechanically heretofore by incorporating extremely small particles into the substrate surface during electrodeposition of the substrate so that the subsequently electrodeposited chromium forms a porous layer interrupted at the site of each particle in the substrate surface. A similar mechanical approach has involved lightly impinging particles of sand or the like on the chromium surface after its electrodeposition to produce micropores.
The subject invention is directed to a cathodic treatment of chromium plated substrates to produce micropores in the chromium plate layer. A review of the prior art has failed to reveal any method of developing by any electric treatment micropores in a chromium plate layer after electrodeposition of the chromium is complete.
U.S. Pat. No. 2,746,915 describes a system for the electrodeposition on a plated surface of a chromium compound coating which is anodic to steel. The coating is applied by placing the article to be treated in a bath, preferably acidic, containing hexavalent chromium and electrolyzing the bath with the article as the cathode. The outer plate layer on the article prior to treatment was, for example, a chromium plate layer about 0.00001 inch thick or about 0.01 mil. The specific treatment described made use of a bath having a hexavalent chromium concentration of 50 grams per liter and a pH of 4.5, a current density of 3 to 4 amperes per square foot, a temperature of 200.degree. F. and a treatment time of 1 to 2 minutes. Current densities of 0.1 to 15 amperes per square foot are mentioned as are a pH as low as 2.5 and hexavalent chromium concentrations of from 1 to 200 grams per liter. The only acid disclosed is chromic acid. There is no mention of micropores or their formation.
U.S. Pat. No. 3,816,082 describes a system of electrochemically treating an article covered with a chromium plate of 0.1 to 0.5 microinch or 0.0001 to 0.0005 mil as a cathode in an aqueous electrolyte containing a water soluble hexavalent chromium compound, preferably 20 to 50 grams per liter of chromic acid, to deposit a metallic chromium and chromium oxide containing film thereon. The electrolyte can also contain a sulfate ion catalyst in an amount of 0.05 to 0.2 gram per liter. The treatment makes use of a temperature of 90.degree. to 150.degree. F., a current density of 25 to 400 amperes per square foot and a treatment time of 0.3 to 0.5 second. However, the chromium must be plated on a zinc substrate. There is no mention of micropores or their formation.
In an article entitled "Process for Coating Tin-Free Steel with Layers of Metallic Chromium and Chromium Oxide," Fukuda et al., Journal of the Electrochemical Society, March, 1969, various plating solutions of chromic acid and sulfate ion are shown for the cathodic treatment of steel. While chromic acid has been in common use for metal finishing, no one has heretofore used chromic acid in any method of making micropores in chromium.
In order for microcracks and/or micropores in chromium plate on a suitable substrate to produce evenly a significant improvement in corrosion protection of the base it is necessary that such microcracks and micropores be distributed over the entire chromium plate surface. Otherwise, non- or lightly-cracked and/or non- or lightly-porous areas receive severe corrosion. For example, in the electrodeposition of microcracked chromium from a fluosilicate-containing bath on a suitable substrate it has been found that the desired microcracks do not tend to form in low current density areas apparently because the thickness of chromium is insufficient, and therefore severe corrosion will occur in these areas. This tendency is offset substantially by the cathodic treatment of this invention which induces the formation of micropores uniformly in the chromium layer including such low current density plating areas.